1. Field of the Invention
The present invention relates to a heat sink for heat dissipation of devices such as semiconductors and the manufacturing process thereof.
2. Description of the Background Art
Heat sinks are generally used as a means to cool semiconductors and other similar devices. Such heat sinks must instantaneously dissipate the heat that is rapidly produced in an element of semiconductors and other similar devices. To meet this requirement it is effective to use materials having a high heat dissipation property, which depends on the thermal conductivity and a specific heat thereof. Copper was once considered an appropriate candidate material because of its relatively high thermal conductivity of 398 W/mK and high specific gravity. However, copper has a coefficient of thermal expansion as high as 17 ppm/.degree. C. compared with that of semiconductors and ceramics for insulation; for example, 4.2 ppm/.degree. C. for silicon and 6.7 ppm/.degree. C. for GaAs. Consequently, when copper is joined with a semiconductor, due to the discrepancy in thermal expansion in the two components when the joined body is subjected to temperature variations both at the time of joining and during operation, too large a thermal stress is applied to the semiconductor, making this combination impractical in many instances.
Another idea is to use an alloy of copper and a metal having a small coefficient of thermal expansion such as tungsten or molybdenum. An alloy or dispersed body such as Cu--W or Cu--Mo system is used because the coefficient of thermal expansion of these materials is similar to that of a semiconductor. However, tungsten and molybdenum have a low thermal conductivity so that their alloys with copper exhibit conductivity no more then about 200 W/mK, which is less than desirable.
Another idea is to use a combination of metal and diamond, which has a high thermal conductivity. A variety of techniques stemming from this idea is disclosed in U.S. Pat. Nos. 5,045,972 and 5,130,771 and unexamined published Japanese patent applications Tokukaihei 3-9552 and Tokukaihei 4-231436. All these techniques take advantage of the high thermal conductivity of diamond in that by embedding it into metal and adjusting its volume ratio to bring the coefficient of thermal expansion of the total system closer to that of a semiconductor so that a thermal distortion caused by the difference in expansion between the two components will be eliminated when the joined body is subjected to temperature variations.
However, when diamond particles are simply embedded into a metal such as Cu, Ag, Au, and Al, due to poor bonding of diamond with these metals, the mechanical strength as a heat sink is intolerably weak and mixing cannot be performed thoroughly during the manufacturing process. To solve these problems, there is another idea, in which the surface of the diamond particles is coated with metal by special means before sintering with a metal such as Cu, Ag, Au, and Al. Although this method provides easy forming and enables the controlling of the composition, the final product manufactured with a conventional metal sintering process through mixing of powder, compacting under pressure, and sintering shows a thermal conductivity of no more than that of the metal used. In yet another method to cope with this problem, before embedding in a molten metal, diamond particles are coated with a metal capable of forming carbide with diamond. This method, although apparently similar to the invention, is not considered a solution to the problem because the metal used to produce the carbide remains around the diamond particles so that the excellent thermal conductivity of diamond cannot be fully utilized.